Malfunctioning of the Injection System
Malfunctioning of the Injection System
The fuel injection system is the most intricate of the systems in a diesel engine. Since the function of an injection system is to deliver fuel to the cylinder at a high pressure, at the proper time, in the proper quantity, and properly atomized, special care and precautions must be taken in making adjustments and repairs.
HIGH-PRESSURE PUMP.-If a high-pressure pump in a fuel injection system becomes inoperative, an engine may fail to start. Information on the causes and remedies for an inoperative pump can be found in the manufacturer’s technical manual. Any ship using fuel injection equipment should have available copies of the applicable manufacturer’s technical manual.
TIMING.-Regardless of the installation or the type of fuel injection system used, the timing of the injection system must be correct to obtain maximum energy from the fuel. Early or late injection timing may prevent an engine from starting. Operation will be uneven and vibration will be greater than usual.
If fuel enters a cylinder too early, detonation generally occurs, causing the gas pressure to rise too rapidly before the piston reaches top dead center. This in turn causes a loss of power and high combustion pressure. Low exhaust temperature may be an indication that fuel injection is too early.
If fuel is injected too late in the engine cycle, overheating, lowered firing pressure, smoky exhaust, high exhaust temperature, or loss of power may occur. Follow the instructions in the manufacturer’s technical manual to correct an improperly timed injection system.
Insufficient Compression
Proper compression pressures are essential if a diesel engine is to operate satisfactorily. Insufficient compression may cause an engine to fail to start. If you suspect low pressure as the reason, check the compression with the appropriate instrument. If the test indicates pressures below standard, disassembly is required for complete inspection and correction.
Inoperative Engine Governor
There are many troubles that may cause a governor to become inoperative. The most frequent trouble associated with starting an engine is generally caused by bound control linkage or, if the governor is hydraulic, by low oil level. Whether the governor is mechanical or hydraulic, binding of linkage is generally due to distorted, misaligned, defective, or dirty parts. If you suspect binding, move the linkage and governor parts by hand and check their movement. Eliminate any undue stiffness or sluggishness in the movement of the linkage.
Low oil level in hydraulic governors may be caused by oil leaking from the governor or failure to maintain the proper oil level. Leakage of oil from a governor can generally be traced to a faulty oil seal on the drive shaft or power piston rod, or to a poor gasket seal between parts of the governor case.
Check the condition of the oil seals if oil must be added too frequently to governors with independent oil supplies. Oil seal leakage may or may not be visible on external surfaces. There will be no external sign if leakage occurs through the seal around the drive shaft, while leakage through the seal around the power piston will be visible.
Oil seals must be kept clean and pliable. Store them properly so they do not become dirty or dry and brittle. Leaky oil seals cannot be repaired. They must be replaced. You can prevent some leakage troubles simply by following proper installation and storage instructions for the seals.
Most manufacturer’s technical manuals supply information on the governor. Special hydraulic governor maintenance manuals made available by the Naval Sea Systems Command are the Marquette Governor Manual, NAVSHIPS 341-5505 (0341-LP-550-5000), and the Woodward Governor Manual, NAVSHIPS 341-5017 (0341-LP-501-7000).
Inoperative Overspeed Safety Devices
Overspeed safety devices are designed to shut off fuel or air in case of excessive engine speed. These devices must be maintained in operable condition at all times. Inoperative overspeed devices may also cause an engine not to start. They may be inoperative because of improper adjustment, faulty linkage, or a broken spring, or the overspeed device may have been accidentally tripped during the attempt to start the engine.
If the overspeed device fails to operate when the engine overspeeds, the engine may be secured by manually cutting off the fuel oil or the air supply to the engine. Most engines have special devices or valves to cut off the air or fuel in an emergency.
Insufficient Cranking Speed
If the engine cranks slowly, the necessary compression temperature cannot be reached. Low starting air pressure may be the cause of such trouble. Slow cranking speed may also be the result of an increase in the viscosity of the lubricating oil. This trouble occurs during periods when the air temperature is lower than usual. The oil specified for use during normal operation and temperature is not generally suitable for cold climate operation.
